1. Field of the Invention
The present invention relates to a polarizing beam splitter and an optical apparatus including the polarizing beam splitter. More specifically, the present invention relates to a photographic optical system, a projection optical system (a projector), an image processing apparatus, a semiconductor manufacturing apparatus, an optical disk recording/reproduction apparatus (an optical pickup apparatus), and other various optical devices.
2. Description of the Related Art
A conventional polarizing beam splitter includes two types of media, which are different in refractive index and cooperatively constitute a dielectric multilayered film configured to split polarized light.
FIG. 2 illustrates a conventional polarizing beam splitter using prisms. As illustrated in FIG. 2, a polarizing beam splitter (PBS) 21 includes a dielectric multilayered film 22, which transmits p-polarized light (one of polarized beams) incident thereon at Brewster's angle. On the other hand, s-polarized light interferes with the dielectric multilayered film 22 and is reflected by the dielectric multilayered film 22.
In general, the following formula (1) defines Brewster's angle θb where n1 represents the refractive index of a medium positioned on the incident side and n2 represents the refractive index of a medium positioned on the exit side in the condition where two neighboring media are present via an interface.tanθb=n2/n1   (1)
To separate polarized light, a medium constituting a prism 23 and a plurality types of media forming the dielectric multilayered film 22 (e.g., a higher refractive index layer (H layer) and a lower refractive index layer (L layer)) are required to satisfy the above-mentioned relationship.
To this end, the relationship defined by the following formula (2) is required to be satisfied, where np represents the refractive index of the medium constituting the prism 23, nH represents the refractive index of the higher refractive index layer constituting the dielectric multilayered film 22, and nL represents the refractive index of the lower refractive index layer constituting the dielectric multilayered film 22.
                              n          p                =                                                            n                H                2                            +                              n                L                2                                                                    sin                2                            ⁢                                                θ                  b                                ⁡                                  (                                                            n                      H                      2                                        +                                          n                      L                      2                                                        )                                                                                        (        2        )            
The dielectric multilayered film 22 functions as a reflection film, which causes s-polarized light to be reflected by an interface between a medium H of the high refractive index layer and a medium L of the low refractive index layer. The dielectric multilayered film 22 includes 20 to 40 layers, which cooperatively constitute a reflection film operable in the entire visible light range.
Regarding s-polarized light, the dielectric multilayered film 22 can widen angle characteristics and wavelength characteristics by increasing the number of layers constituting the multilayered film.
On the other hand, as discussed in U.S. Pat. No. 5,042,925, a polarizing beam splitter can include an adhesive layer having optical anisotropy and being sandwiched between two prisms, instead of using a dielectric multilayered film.
The difference in refractive index between birefringent materials is a key factor in operation of the polarizing beam splitter discussed in U.S. Pat. No.5,042,925. In this case, although the refractive index difference is small, if the incident angle of light is 60° or a sufficiently large angle, the polarizing beam splitter can totally reflect one of two polarized beams selectively.
To ensure total reflection, it is required to set the incident angle of light to be equal to or greater than a critical angle θc. Where n1 represents the refractive index of the incident side medium and n2 represents the refractive index of the exit-side medium, the following formula (3) defines the critical angle θc.sinθc=n2/n1   (3)
On the other hand, as discussed in U.S. Pat. No. 7,116,478, a polarizing beam splitter can use a grating structure having a periodic pattern equivalent to or less than the wavelength of light.
According to the polarizing beam splitter discussed in U.S. Pat. No. 7,116,478, a polarized beam splitting layer includes a structured layer, in which a plurality of gratings parallel to a first direction are periodically disposed in a second direction orthogonal to the first direction.
The polarizing beam splitter splits polarized light into a polarized light component of the first direction and a polarized light component of the second direction by reflecting and transmitting the light.
The structured layer discussed in U.S. Pat. No. 7,116,478, which is characterized by a periodic arrangement of gratings disposed at intervals equivalent to or less than the wavelength of light, is generally referred to as a “subwavelength periodic structure (SWS).”
Because of a minute structure characterized by intervals (periods) shorter than the wavelength of light, the structure of SWS is not recognizable, and, therefore, SWS has characteristics similar to a uniform medium. Therefore, SWS can function as a layer having an equivalent refractive index determined by refractive indices of constituent materials and the ratio of them. Furthermore, SWS can function as a layer having strong anisotropy if it is configured to have a periodic structure differentiated in each direction.
According to U.S. Pat. No. 7,116,478, a large difference in refractive index can be provided between s-polarized light and p-polarized light which are incident on a surface of the polarizing beam splitter using SWS. Therefore, the polarizing beam splitter can totally reflect only one of the polarized beams. In other words, the polarizing beam splitter can easily satisfy a required reflection condition.
For example, if gratings of SWS (structured layer) are periodic in a direction parallel to an incident plane, a polarized beam splitting layer can totally reflect p-polarized light and transmit s-polarized light. Furthermore, the polarized light that is totally reflected is adjustable by changing the periodic direction of the structure.
The structure discussed in U.S. Pat. No. 7,116,478 is superior, in incident angle characteristics, to a polarizing beam splitter including a dielectric multilayered film utilizing the Brewster's angle.
A general polarizing beam splitter including a dielectric multilayered film, which causes optical interference, utilizes the Brewster's angle for splitting polarized light. Therefore, both incident angle characteristics and broadband characteristics tend to be inferior. Furthermore, if design is prioritized for characteristics of one polarized light, the design will adversely influence characteristics of the other polarized light. Thus, if required to satisfy both requirements, the design becomes very complicated.
The polarizing beam splitter having the SWS structure discussed in U.S. Pat. No. 7,116,478 can effectively split the incident light into s-polarized light and p-polarized light.
However, the polarizing beam splitter discussed in U.S. Pat. No. 7,116,478 tends to generate a large change in incident angle characteristics (characteristics in a relationship between incident angle and reflectance).
Therefore, there is a tendency that, if a polarizing beam splitter is incorporated in an optical pickup apparatus, it becomes difficult to detect the quantity of light entering a monitor photo-detector to adjust the quantity of light entering an optical disk.
Moreover, it is difficult to arbitrarily adjust the characteristics of a polarizing beam splitter for a predetermined polarized light component incident thereon in a wide wavelength range to adequately maintain broadband characteristics.